Science Interviews

Interview

Sun, 16th Dec 2007

The Science of Snowflakes

Professor Kenneth Libbrecht

Listen Now    Download as mp3 from the show The Naked Scientists Christmas Special

natural snow crystalMeera - Itís that time of year again, when weíre all secretly hoping to wake up on Christmas morning to see a nice layer of white coating our Christmas streets. Thatís right, the pleasurable sight of snow that makes the cold so much more bearable. But have you ever thought about the structure of the snowflakes that actually make up some of that snow and how those intricate patterns were created that we see printed in our Christmas cards every year? I spoke to Professor Kenneth Libbrecht at Caltech University and picked his brains about what snowflakes actually are.

Kenneth - Theyíre basically made of ice. What makes them unique is they form from water vapour in the air. When water vapour is condensing into solid ice it forms beautiful patterns.

Meera - Snowflakes form in a really distinctive way. Thatís how we all recognize them. Why do they form like that?

Kenneth - What makes them interesting is theyíre a very complex shape but theyíre still symmetrical. The real reason that works is the growth of the crystal is very sensitive to temperature and humidity. When the flake is falling and itís forming in the atmosphere they will start out growing into a small hexagon because of the way the water molecules hook up. The crystal lattice will form a small hexagon. Then the corners of the hexagon kick out a little further into the air. Theyíll tend to grow a little faster and the crystal can develop branches. Then as the branches grow, their growth is very sensitive to the conditions of seed. The crystalís falling through the cloud the temperature of the seed changes slightly all the time with the humidity. Even very small changes can change the way the crystal grows. As this thing is falling the growth of each arm will be very complex because of the path it takes through the cloud. The final shape of an arm reflects the whole history of its growth. Each arm has the same history as theyíre all connected together. Each arm grows, more or less, in synchrony and what you end up with is something very complex yet still has this six-fold symmetry.

Meera - But can you believe there are over forty different classes of snowflakes? What causes these differences?

Kenneth - They really grow in a remarkable variety of different types: columns, branch structures and one of my favourites is a capped column Ė where thereís a column with plates on either end. They all grow at different temperatures and the growth can oscillate between plates and columns as a functional temperature. Plate-like crystals are just below freezing and then columns are a little colder still than the plates. No one quite understands why the crystals grow that way but thatís what they do. The conditions do vary a lot in the atmosphere so you get lots of different types.

Meera - The actual change in temperature to create these structures is actually really small, sometimes even just one or two degrees.

Kenneth - Thatís right. The plates grow around -2 Celsius and columns at -5 and plates again at -15.

Meera - So with just a few degrees causing all these changes, which temperature gives us the best looking snowflake?

Kenneth - Well, the really nice-looking crystals tend to grow when itís pretty cold, around -15 Celsius. Those are these large stellar dendrites (stellar crystals): the ones you really associate with crystals when you think of them. Theyíre very thin plates and they have beautiful branches and lots of structure.

Meera - What about the myth that there arenít any two snowflakes alike?

Kenneth - When you are growing a snowflake the growth is so sensitive to temperature that it tends to form a lot of different possible shapes. If you add up the number of possible ways of making a snowflake you usually find that itís far greater than the total number of atoms in the universe. So itís fair to say that if you go out looking youíll never find two that are exactly alike.

Meera - To finish off, I asked Kenneth for some tips on how we can make the most out of any snow we get this Christmas.

Kenneth - Itís really fun to go looking for snowflakes. You donít need any real equipment. A little magnifying glass should help. You can even see quite a bit with the naked eye. Youíll find all sorts of different shapes. One of the places I like best is the windshield of a cold, parked car. It has a nice slope and you can brush the crystals away and look at them. Better make sure itís your own car though!

Meera - So there you go. A nice family activity to do together over the Christmas period. Thatís if we manage to get any snow. To see the different classes of snowflakes to aid you in your snowflake spotting, or simply just to find out more information about these structures, you can go online to Kennethís website at www.snowcrystals.com

Kat - That was Naked Scientist, Meera Senthilingham talking to Kenneth Libbrecht at Caltech University. If you want to find out more about snowflakes and the physics behind their design we do have an article on our website. To find it just go to www.thenakedscientists.com/articles

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